Twelve morbidly obese patients had jejunoileal bypass surgery and serial detailed pulmonary function studies done over a one year period. Before operation they showed evidence of a decreased ventilation: perfusion ratio producing a "shuntlike effect" (Pao2 = 73, Paco2 = 37, pH = 7.44) while breathing room air (FIo2 = 0.209). They also had measurements consistent with pure restrictive lung disease. There was a highly significant improvement in the arterial oxygenation over the entire one year period and a significant loss of chest wall mass over the first 6 month period. The improved oxygenation most likely was due to a return of lung volumes toward normal and an improvement in the efficiency of the respiratory muscles.